Thanks a lot Carl...this is very enlightening. If I could impose one last question before I go off and digest everything:
What is the "correct" approach to capturing the runtime values of any references that may be bound by the enclosing environment, for splicing into the final recursively-expanded expression? ;; example 1 (define y 3) (define x 3) (define-dsl-syntax (pred? x) (or (= 2 x) (> x y))) (define-dsl-syntax (bad-pred? x) (or (= 2 x) (> x z))) (recursive-expand (pred? 1)) '(or (= 2 1) (> 1 3)) (recursive-expand (bad-pred? 1)) >> unbound identifier: z My naive approach was to collect all the identifiers in the expression body that had bindings, compare them to the argument list of the macro with bound-identifier=? to see which ones were explicitly introduced by the user, and then eval the remaining ones at runtime in a second step in order to splice them in. I haven't tried this, and am sure people are cringing just by reading it, lol. I know there are tons of features like marking and syntax properties and origins, etc which I don't yet understand, and which may provide a more durable solution. If needbe, I could explicitly provide to the macro the bindings I wish to capture, like postgresql does with query params...ie, (define-dsl-syntax (pred? x) (or (= 2 x) (> x $1)) #:capture (y)) ;; or something but for obvious reasons it is much better if these expressions just expanded and automatically captured any referenced values in the same way as would happen at runtime. Thanks a lot. Scott. On Sat, Jan 25, 2014 at 6:00 PM, Carl Eastlund <carl.eastl...@gmail.com>wrote: > Scott, > > I see what you're doing now. You're not actually trying to use macro > expansion at all; you're just using local-expand to substitute the > definition of pred? where it occurs, so that you can make its macro > definition also serve as its DSL definition. That's sensible, but > local-expand is still doing more than you want it to. That's why I put in > all the expansion caveats -- not because you necessarily meant to do full > expansion, but because local-expand is pretty explicitly built for full > expansion, and always tries to push as far as it can. Any time the caveats > about expansion don't apply, local-expand is probably a bigger gun than you > need. > > Where local-expand is going to bite you is when the definition of pred? > uses a macro at its top level. For instance: > > (define-syntax-rule (pred? x) (or (< x 3) (> x 7))) > > Here, local-expand is going to expand the use of (or ...), and any macro > that (or ...) produces at its top level, until you reach a core form as the > main expression, or something you've put in an explicit stop list. That's > not what you want, as I understand it -- you only want to expand pred?. > > So what to do when you want to apply one macro, but not perform general > expansion? Extract its transformer using syntax-local-value, and apply it > to the expression. You probably also want to apply a syntax mark before > and after transformation, just to simulate the base level of hygiene the > macro may be relying on. It might not be necessary for simple definitions, > but it can't hurt. > > I wrote up some code that does this, along with a test showing that it > won't expand "or" too far. It's also reasonably hygienic -- it won't be > confused if someone defines a different macro named "pred?", for example. > I don't know if that's a concern, but again, it can't hurt. Anyway, you > can find what I wrote here: https://gist.github.com/carl-eastlund/8626893 > > Carl Eastlund > > On Fri, Jan 24, 2014 at 1:30 PM, Scott Klarenbach <sc...@pointyhat.ca>wrote: > >> Just an update, I was able to make this work. >> >> #lang racket >> (require (for-syntax racket/syntax syntax/stx)) >> >> (define-syntax-rule (pred? x) (> 3 x)) >> >> (define-for-syntax (recursive-expand stx) >> (let loop ([l (syntax->list stx)]) >> (cond [(stx-null? l) l] >> [(stx-pair? (stx-car l)) >> (cons (loop (stx-car l)) (loop (stx-cdr l)))] >> [(equal? 'pred? (syntax->datum (stx-car l))) >> (local-expand (cons (stx-car l) (loop (stx-cdr l))) 'expression #f)] >> ;; this works >> [else >> (cons (stx-car l) (loop (stx-cdr l)))]))) >> >> (define-syntax (test stx) >> (syntax-case stx () >> [(_ x) >> (with-syntax ([expanded (recursive-expand #'x)]) >> #''expanded)])) >> >> (module+ test >> (require rackunit) >> (check-equal? (test (or (< 10 x) (pred? y))) >> '(or (< 10 x) (> 3 y)))) >> >> The code I couldn't figure out last night was: >> (local-expand (cons (stx-car l) (loop (stx-cdr l))) 'expression #f)] >> >> Thanks. >> >> -- >> Talk to you soon, >> >> Scott Klarenbach >> >> PointyHat Software Corp. >> www.pointyhat.ca >> p 604-568-4280 >> e sc...@pointyhat.ca >> 200-1575 W. Georgia >> Vancouver, BC V6G2V3 >> >> _______________________________________ >> To iterate is human; to recur, divine >> > > -- Talk to you soon, Scott Klarenbach PointyHat Software Corp. www.pointyhat.ca p 604-568-4280 e sc...@pointyhat.ca 200-1575 W. Georgia Vancouver, BC V6G2V3 _______________________________________ To iterate is human; to recur, divine
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